Nitinol is a nickel-titanium intermetallic compound invented at the Naval Ordinance Laboratory in the early 1960""s. It is a material with useful properties, but manufacturers who have worked with it have had little success in making Nitinol parts and semi-finished forms. Because Nitinol is so extremely difficult to form and machine, workers in the metal products arts usually abandoned the effort to make products out of anything except drawn wire because the time and costs involved did not warrant the paltry results they were able to obtain.
Nitinol, particularly Type 60 Nitinol (60% Nickel and 40% Titanium by weight), has many properties that are unrecognized as of potential value. It can be polished to an extremely smooth finish, less than 1 microinch rms. It is naturally hard and can be heat treated to a hardness on the order of 62Rc or higher. It can be processed to have a very hard integral ceramic surface that can itself be polished to an even smoother surface than the parent metal. It is non-magnetic, immune to corrosion from most common corrosive agents, and has high yield strength and toughness, even at elevated temperatures. It is 26% lower density than steel for weight sensitive applications such as aircraft, satellites and spacecraft. However, there has hitherto been little effort in making useful parts out to Nitinol because it is so difficult to work, because it was known to be brittle, and because there has been no known method to make parts and forms out of Type 60 Nitinol.
Accordingly, this invention provides several processes for making Nitinol parts and forms to net and near net size, and for producing the desired mechanical properties of hardness, toughness, and elasticity and shape memory effect in those parts and forms.
The invention includes processes for manufacturing Nitinol products, and the Nitinol products made by the processes. The processes include making forms and parts by friable mold casting or die casting using molten Nitinol poured or injected into the mold or die. After the Nitinol has cooled to a solid state, it is ejected from the die or removed from the mold by disintegrating the friable material of the mold and is heated to an elevated temperature under high pressure to consolidate the Nitinol and remove any internal voids.
Parts and forms made from rolled or cast Nitinol may be heat treated to reduce brittleness and improve toughness and impact strength, and give the parts and forms made of Type 60 Nitinol a highly elastic property which I am calling xe2x80x9cultraelasticityxe2x80x9d.
The part may be hot machined to reduce it to near net size, and may be ground to reduce the part to the exact specified part size. For example, cylindrical parts can be centerless ground; balls can be ground in a conventional ball grinder; flat stock can be surface ground. For parts requiring a smooth surface finish, polishing or lapping provides the specified surface finish on the part, down to 0.5 microinch RMS or finer. The part may be heat treated to obtain the desired hardness, from RC40 to RC65.
An integral surface oxide of any of several colors can be formed on the surface of the part. The oxide surface may itself be polished to an even finer surface finish. These process elements may all be used to produce a particular part that requires the characteristics provided by each process element, and they may be used in combinations that omit particular process elements or substitute others to give the desired characteristics of the part.
Shape memory effect in Type 60 Nitinol, never before known to exist, may be obtained by heat treating to about 675xc2x0 C.-700xc2x0 C. and then cooling slowly over 8-10 hours in the oven.